1. Field of the Invention
This invention relates to steel cords used as a reinforcement for rubber articles such as tire, industrial belts and the like as well as plastic products having a poor fluidity due to the mixing with fine powders such as carbon black, silica and so on, and more particularly to an improvement of steel cords having a so-called open twisted structure with the use of steel filaments.
2. Related Art Statement
In articles or products reinforced with steel cord, there is a problem that the durable life of the product is lowered due to the corrosion of steel filament under an influence of water penetrates into the product. For this end, it has been attempted to improve the corrosion resistance of the steel cord by subjecting the surface of the steel filament to a metallic plating, or by forming a so-called open twisted structure capable of penetrating rubber into the inside of the cord (Research Disclosure, June 1978, page 33).
In the steel cords using metallic plated steel filaments, when they are used under a condition that strain is repeatedly applied to the steel cord, the plated layer is abraded by fretting between mutual steel filaments in the cord to lose the corrosion resisting action. On the other hand, in the cord of the open twisted structure, the space between mutual steel filaments becomes narrower when a slight tension is only applied to the cord, and consequently the penetration of rubber or plastic into the inside of the cord becomes difficult.
In order to improve the rubber penetrability, Japanese Utility Model laid open No. 61-108397 discloses steel cords having such an open twisted structure such that steel filaments are substantially twisted in an ellipsoidally helical shape at the section of the cord. Since this cord is large in the tensile elongation under a load of 2 kg, however, the space between mutual steel filaments constituting the cord becomes narrower and the penetration of rubber or plastic into the inside of the cord is obstructed.
Furthermore, Japanese Patent laid open No. 62-170594 discloses the open twisted structure that the steel filaments are divided into at least two groups having different forming ratios and a maximum forming ratio is 1.65, minimum forming ratio is 1.05 and a difference therebetween is within a range of 0.20-0.40. In this case, however, the helical shape of steel filament is not optimized, so that the processability of the cord and the rubber penetrability are undesirably deteriorated and also the harmonization of steel filaments and the retention of strength in the cord are degraded.